1. Field of the Invention
This invention concerns grinding machines and more particularly gear grinding machines of the type specifically adapted to crown gear teeth.
2. Description of the Prior Art
Crowning of gear teeth is commonly done to improve the contact between meshing gears where some misalignment is anticipated since misaligned gears can produce a line contact between the mating gears increasing the tooth contact stress and thus also increasing wear and noise. The "Crowning" of gear teeth is the curving of the tooth flanks along their lengths. The usual approach in producing gear crowns as typified in the disclosure in U.S. Pat. No. 2,325,836 to Drummond is to cam the wheel head, i.e., that portion of the grinding machine in which the grinding wheel is supported so as to cause the grinding wheel to be moved toward or away from the gear teeth as the grinding wheel is moved across the gear teeth with grinding wheel being profiled so as to produce the appropriate crowning. This approach will generally be satisfactory for normal applications. However, for high precision crowning which is commonly required in such applications as aerospace components, the mass which is shifted during the camming translation of the wheel head levels inevitably to excessive deflections, backlash and other factors which in turn lead to relative inaccuracy and nonrepeatability of results.
Some arrangements have also been provided in which the grinding wheel is mounted on a pivotal fixture, described in U.S. Pat. No. 2,490,848 to Vossler and No. 2,392,819 to Gruenberg et al., but these arrangements are not adapted to crowning of external and internal gears and in addition the mechanical arrangements disclosed also involve the movement of a considerable mass by virtue of the provision of a drive motor carried by the pivoted fixture. In addition, the mechanical arrangements disclosed involve numerous mating parts which lead to considerable clearance induced lost motion and excessive deflection such that high precision results are not likely using such apparatus.
A further problem in obtaining precision crownings is the inducement of minor vibrations created by an overhung grinding wheel mounted on a pivoted arm, which are accentuated by the support of relatively large masses on the pivotal frame and also the non-damping characteristics of the cast iron or steel material from which such components are manufactured. These vibrations would likewise adversely effect the precision with which the grinding process could be carried out.
Commonly, the gear crowning mechanisms which have been utilized in the past have entailed major modifications to the existing gear grinding machines and which in some instances the fixturing required must be removed in order to perform ordinary gear grinding processes. It would, of course, be advantageous if the gear grinding fixture could be provided which converts a conventional gear grinding machine into a machining which has a capability for high precision crowning operations and does not effect the ability of the machine to carry out normal gear grinding in which the wheel is drawn in a lineal path across the gear teeth.
It would, of course, also be advantageous if such fixturing could be provided which is of relative simple design such that it may be trouble-free and inexpensive to manufacture.
An additional drawback of some of the gear crowning mechanisms which have heretofore been utilized is that asymmetrical crowning cannot be obtained due to the nature of the crowning process. This drawback is inherent in those machines in which the gear is rocked as the grinding wheel passes along the length of the gear tooth. In many applications, such as in planetary gear sets, the ring gear is in overhung engagement with the pinion gears and, in this instance, the crowning would preferably be asymmetrical toward the position of the pinion gear and thus it would be desirable that any such gear crowning machine would have the capability of crowning the tooth flanks in any desired configuration.
Accordingly, it is an object of the present invention to produce a gear crown grinding machine which is capable of producing highly precise results by virtue of fixturing which produces a minimum deflection and vibration as the process is carried out, by reducing the mass which is moved during the crowning process and maximizing the rigidity of the parts by minimizing the number of mechanical elements which are stressed.
It is a further object of the present invention to provide a fixturing which may be relatively simply adapted to existing gear grinding machines to afford the same capability of carrying out precision crowning operations and also allowing the continued use of the gear grinding machine as a gear grinder.
It is yet another object of the present invention to provide such a machine in which vibrations of the fixturing is held to a minimum to further enhance the degree of precision obtainable.
It is yet another object of the present invention to provide such a machine in which gear crowning may be carried out on internal or external gears and in which the crowning shape may be formed asymmetrically with respect to the gear tooth length.
It is still another object of the present invention to provide an arrangement which is relatively simple in configuration and in its adaptation to existing gear grinding machines.
It is yet another object of the present invention to provide such a fixturing in which the friction forces involved are at a minimum to reduce deflections occurring in the fixturing as a result of frictional forces borne by the various structural components.